This invention relates generally to radio frequency amplifiers and more particularly to amplifiers of such type which include slow wave delay line structures.
As is known in the art, radio frequency amplifiers have a wide range of applications. One type of such, amplifier includes a slow wave delay line structure wherein as an applied radio frequency energy signal propagates down the slow wave delay line structure, the energy therein interacts with an adjacent electron beam in such a way that a portion of the energy in the electron beam is transferred to the propagating wave with the result that the radio frequency energy emerging from the delay line structure is amplified. One type of such amplifier is a travelling wave tube (TWT) amplifier. Here, an electron gun produces a pencil-like beam of electrons having a velocity that typically corresponds to an accelerating voltage of the order of 10 kilovolts. The beam is typically directed from a cathode through a long, loosely wound electrically conductive helix wire, which provides the slow wave delay line structure, to a collector. An axial magnetic focusing field, either uniform or periodic is provided to prevent the beam from spreading and to guide it through the center of the helix. The radio frequency energy signal is applied to the end of the helix wire adjacent the cathode and the amplified signal then appears at the end of the helix wire adjacent the collector. The applied signal propagates around the turns of the helix wire and produces an electric field at the center of the helix that is directed along the helix axis. Since the velocity with which the signal propagates along the helix wire is approximately the velocity of light, the electric field produced by the applied signal advances at a velocity slower than the velocity of light; i.e. it advances at the velocity that is approximately the velocity of light multiplied by the ratio of the helix wire pitch to the helix wire circumference. When the velocity of the electrons in the beam travelling through the helix wire approximates the velocity of the signal propagating axially along the slow wave helix structure, an interaction takes place between the moving signal or wave produced by the electric field, and the moving electrons which is of such a character that on the average, the electrons in the beam deliver energy to the propagating signal on the helix wire. This causes the signal on the helix wire to become amplified at the output end of the helix wire.
As is also known in the art, various support structures have been used to support the helix wire within the TWT envelope. One type of support structure includes the use of a plurality of dielectric support rods, such as those described in U.S. Pat. No. 3,778,665, issued Dec. 11, 1973, inventors Robert Harper and David Zavadil, and assigned to the same assignee as the present invention. More particularly, the TWT includes a hermetically sealed, elongated, cylindrically shaped envelope. Coaxially disposed within the cylindrical envelope is the helix wire. A plurality, typically 3, symmetrically spaced elongated dielectric rods which extend longitudinally parallel to the common axis of the cylindrical envelope and the helix wire are provided. The rods are of a dielectric material so as to electrically insulate the helix wire from the envelope or ground of the TWT and thereby prevent short circuiting of the applied radio frequency energy signal. The rods have a generally rectangularly shaped cross-section in a plane perpendicular to the common axis. The rods are wedged between inner surface portions of the cylindrically shaped envelope and outer peripheral portions of the helix wire to thereby support the helix wire coaxially aligned within, but electrically insulated from, the elongated cylindrically shaped envelope. The helix, slow wave delay line structure, due to its ohmic resistance as well as electron bombardment, is required to dissipate a considerable amount of thermal energy during the interaction process. Thus, while it is required that the support rods are of dielectric material they must have high thermal conductivity. Typical prior art devices utilize slow wave support structures of nonelectrically conductive but thermally conductive materials such as beryllia, boron nitride, or other ceramics having high thermal conductivity characteristics.
As is further known in the art, the dielectric support rods are susceptible of becoming electrically charged when stray electrons from the electron beam strike them. The resulting charge build-up, if sufficiently large, will cause either the deflection of the electron beam, if unsymmetrical, or act as an electrostatic lens, if symmetrical. This latter phenomenon could increase beam scalloping which could also increase interception by the helix thereby increasing interception current in the helix. Further, rod charging can cause slowing down or deflection of the electron beam, which results in an increase in the current striking the helix wire in a localized area. This can ultimately lead to an excessive rise in the helix wire temperature and ultimately to failure of the tube. Generally, however, a TWT experiencing support rod charging fails due to excessive helix wire interception current.
One method of avoiding this problem is by tedious adjustment of the local magnetic field along the helix wire. This operation, sometimes referred to as shimming, is very time-consuming, since attempts to shim do not always converge to an acceptable result. An additional difficulty is the time taken for the electric charge to build up on the support rods since this may lead to a shimmed tube not performing properly when turned on from a "cold" start.
Another method used to eliminate support rod charging has been to increase the electrical conductivity of the rod surface to prevent the build-up of charge on the rod surface. This approach requires the use of a thin electrically conducting film, such as graphite, on those portions of the rods that are in close proximity to the electron beam, since these portions are in the radio frequency field of the helix they may introduce unwanted loss in the helix circuit. As a consequence, this technique sometimes forces a compromise between achieving a reliable film that is thick enough to prevent rod charging and a film that is not so thick as to introduce radio frequency energy loss.
Finally, as mentioned briefly above, the material typically used for the dielectric helix support rods is boron nitride (BN) or beryllium oxide (BeO). While the beryllium oxide rods do not exhibit rod charging, it is a more difficult material to use from a mechanical fabrication standpoint due to its toxicity and brittleness. Boron nitride, on the other hand, is an easier and more desirable material to use because it is more "forgiving" in its mechanical mating characteristics when it interfaces between the outer peripheral portions of the helix; however, boron nitride does exhibit the aforementioned undesirable rod charging characteristics. Boron nitride also has a lower dielectric constant than beryllia which has electrical advantages.